This invention generally relates to the electric lamp art and has particular reference to a method of improving the optical transmissivity of polycrystalline alumina arc tubes during manufacture, and to arc tubes which are produced by such method.
High-density polycrystalline alumina is a well known translucent ceramic material and a detailed disclosure of the various operations required to manufacture it from compressed finely-divided alumina of high purity in a form suitable for use as light-transmitting arc tubes for high-intensity electric discharge lamps (such as high-pressure sodium vapor lamps) is presented in U.S. Pat. No. 3,026,210 issued Mar. 20, 1962 to R. L. Coble. In accordance with the teachings of this patent, additions of small but effective amounts of magnesia up to 0.5 weight percent are included in the raw-mix composition to improve the optical transparency of the sintered alumina tubing. Since the amount of useful light produced by a high-intensity discharge lamp is inherently controlled by the ability of the arc tube to transmit the visible radiations generated by the discharge, intensive research has been conducted to discover and develop various ways of improving the optical transmission characteristics of arc tubes composed of polycrystalline alumina. In accordance with one prior art approach to this problem, the sintered tubing is subjected to a flux polishing treatment in a bath of molten sodium borate at a temperature in the range of from about 762.degree. C. to 857.degree. C. This arc tube polishing concept is disclosed in U.S. Pat. Nos. 3,935,495 and 4,079,167 issued to G. E. Scott, Jr. et al.
As stated in the aforementioned patents, the flux polishing process reduces the high spots on individual exterior alumina crystals without materially introducing low spots at the grain boundaries with the result that the treated arc tube has a smoother surface and exhibits increased optical transmission. However, the fluxing action leaves a glassy coating on the arc tubes which must be removed by washing them in a dilute mineral acid solution after the tubes have been withdrawn from the molten flux bath and allowed to cool to ambient temperature. Thus, while the flux polishing process achieves the desired objective of increasing the optical transmission characteristics of the arc tubes, it is expensive and time consuming and, as such, is not particularly suitable for use in the manufacture of high-pressure discharge lamps in mass-production quantities.
U.S. Pat. Nos. 4,150,317 and 4,169,875 to H. M. Laska et al. disclose the concept of improving the in-line transmission characteristics of polycrystalline alumina arc tubes by using alumina powder that is very pure (99.99% pure and devoid of grain-growth promoting impurities such as sodium and iron) and also limiting the magnesia content in the raw-mix composition to 0.1% by weight or less. The patents indicate that the reduction in the magnesia constituent prevents the formation of secondary magnesia alumina spinel phase at the grain boundaries in the final sintered product.
It would accordingly be very advantageous from both a cost and manufacturing standpoint if an inexpensive method could be provided for improving the optical transmission characteristics of polycrystalline alumina tubing which would not materially disrupt or delay the normal sequence of operations required to form the tubing from powdered raw materials.